Shot Peening of Ceramics: Damage or Benefit?

Transcription

1 Shot Peening of Ceramics: Damage or Benefit? Tobias Frey, Wulf Pfeiffer Praunhofer Institute for Mechanics of Materials, Tirciburg, Germany I Abstract Non-transformation toughened ceramics show the typical brittle material behavior of failure before deformation at room temperature. Thus, strengthening of ceramics due to deformation induced compressive residual stresses has been thought to be not possible. Nevertheless, preliminary investigations had shown that, using ceramic-specific parameters, shot peening can introduce high compressive residual stresses into the near-surface of silicon nitride and improve the load capacity. The aim of the presented investigation was to improve the shot peening conditions in order to extend the increase of load capacity while maintaining the surface integrity. The materials investigated where alumina and silicon nitride, the properties determined where residual stresses, load capacity and topography. For!.he assessment of the surface strengthening the X-Ray diffraction analysis (XRD) and the ball-on-plate strength test were used. Due to the low penetration depth of X-rays XKD allows to evaluate the peening-induced residual stresses. In the ball-on-plate test, the sample is loaded with a spherical silicon nitride indenter up to failure of the sample which is detected by a high frequency ultrasonic detector. The results show that high compressive residual stresses in the GPa-range can be introduced in silicon nitride and alumina which may boost the load capacity of the near surface layers by a factor of up to 9. Only little eflect on the surface integrity could be obtained. 2 Experimental Details The materials investigated were a co~nmercially available silicon nitride and a commercially available fine-grained alumina. 'The most important material characteristics are given in Table 1. 'PabIe 1: Materials investigated Material Specification Company Young's Characteristic Fracture tough- Ref. n~odulus strength ness Silicon SN-N320X Ceramics For 300 GPa 877 MPa 4.2 MP~ m1i2 Nitride Industry, CFI Alumina A61 Kennametal 390 GPa 400 MPa 4.0 MPa m1i2 [2] Hertel AG [I]

2 2.2 Shot Peening Shot peenmg was performed wlth an ~nject~on system Cemented carblde beads w~th a d~amcte~ of (650 i- 40) pm were shot on the surface of the ceramlcs uslng dfferent pressures between 2 bar and 4 bar and treatmcnt tunes between 280 and 840 seconds The dlstal~ce between the sample and the noz~le were 20 cm All samples wele pollshed usrng a 1 pm drarnond abraslve prlor to the shot peenmg Because of the lower hardness of the Almen strlp ~n relatlonshlp to the ceralnlcs samples the treatment tme were chosen 8 5 and 25 5 tmes longer than leachmg the 98 % coverage of the Alrnen Strp 2.3 Determination of Residual Stresses and Load Capacity The load capaclty of the shot peened and the polished reference samples was determined using the ball-on-plate test. Thc load on a 10 mm silicon nitride ball is increased stepwise until a typical cone-crack appears which follows exactly the maximum tensile slresses. Because of the statistical behavior of ceramics, the load which causes fracture varies from test to test within a certain scatter-band. Typically 17 san~ples with equal surface conditions were tested and the average fracture loads and the corresponding standard deviations were calculated. The stress field of this static ball-on-plate contact is typical for stress fields occurring in contact situation e.g. in roller-bearings. The contact induced stress fields show strong gradients and the tensile stresses, which lead to cracks in brittle materials, are restricted to a very thin surface layer. The residual stress states of the near-surface layers of the samples were measured by X-ray d~ffract~on The mean stless values w~th~n the penetrahon depth 111 the range of 10 pm wele evaluated usmg the so-called ~m~\~~-method [3] The most ~mportant measurement paramete~s are glven III Table 2 Table 2: Parametevs of rcsidual stress evaluations Material Lattice plane Radiation iy-range X-ray elastic constant % s2.- Silicon Nitride (41 1 } CrKa -64" 5 ' " 3.89 GP~' 3 Results 3.1 Residual Stresses and Load Capacities Fig, 1 correlates the near-surface stress states and the load capacities of polished and differently shot peened silicon nitride and alumina samples. The polished silicon nitride reference samples - representing the near-surface condition prior to shot peening - show small compressive residual stresses in the range of 100 MPa and a fracture load of about 3 kn in the ball-on-plate test.

3 residual stress in MPa Figure 1: Fracture load vcssus residual stress of silicon nitriclc and alumina samples in polished and dirferent shot peened conditions Shot peening allowed introducing up to 1.25 Gl'a compressive residual stresses. These compressive stresses shifted the load needed to fracture the surface layers from 3 kn to more than 9 kn, 111 the case of the samples with the highest compressive residual stresses the increase of near-surface strength was so high, that the load limit (9.2 kn) of the ball-on-plate testing device was reached in some cases and the samples passed the test without any crack. Thus, for the most effective shot peening process no error bar could be calculated for the fracture load. The results for similar experiments on alumina are also shown in Fig. 1. As obtained for silicon nitride high compressive residual stresses up to 1.1 GPa were created to the shot peening. These compressive stresses boost the fracture load from 0.7 kn up to more than 9 kn. Again, the Load range of the ball-on-platc testing device was not high enough to introduce Ilertzian cone cracks in all samples treated with the two most effective shot peening conditions. 3.2 Surface Rouglmess Shot peening may influence the surface integrity in two ways. First, each hit by a bead will produce a localized macroscopic deformation accompanied by the creation of dislocations in the near snrface crystallites. These effects are needed to create the strength increasing compressive residual stresses. 'The superposition of ~nany localized deformations will result in an overall roughness of the surface. The surface topography resulting from a shot peening condition which comprises a more or less complete overlay of dips is shown in Fig. 2. In the optical micrograph nearly no difference between the polished reference surface and the shot peened area can be obtained. In addition no significant transfer of debris to the surface is obtained which can be concluded from the natural voids of the material still being visible at the peened surface. Fig. 3 shows the average roughness of the surface for the polished reference sample and after different peening treatments respectively. The shot peening of the surface leads to a small increase of the roughness up to 0.09 yrn for the highest peening pressure of silicon nitride sam-

4 I polished shot peened + Figure 2: Topographical map (top) and optical micrograph near the boarder between a polished and a shot peened (2 bar) area of a silicon nitride sample c,zm s r m e % 004! g 002 Oo0 9 LO..,he, - L-. u 4 bar ool,dmd 2 bar 3 aal 3 bar 4 oar poliihed Figure 3: Average roughness K, of the polished and shot peened silicon nitride and alumina samples ple and 0 14 ym fol the alumma sample One reason f a the higher roughness of the alumina samples could be the lowel hardness compared to the sillcon nmde ceramic 3.3 Evaluation of Single Hits Silicon nitride and alumina samples showed slmilar results concermng the surface ~ntegrity. The area and depth of the localized deformatlon can he evaluated by inspecting single hits on the

5 Figure 4: Topography of a polished silicon nitride surface with dips produced by single hits of'cemented carbide beads a1 maximurn peening pressurc of 4 bar surface. Fig. 4 shows the topography of the most intense shot peening condition with 4 bar peening pressure, Fig. 5 shows the mean values of the diameter and depth of single dips as a function of the peening pressure. The mean values were calculated from the ineasurcment of typically 25 dips per peening pressure. Higher pressures leads to a higher velocity of thc beads and to a larger deformation. The plastic defo~mation is somewhat higher in alumina than in silicon nitride, L. 0 Alumina peening preswre in bar peenlng pressure in bar Figure 5: Iliameter and depth of single hits by cliffcscni peening pressures for silicon nitride and alumina In order to detect a possibly introduced damage due to shot pecning the aluini~la and silicon nitride samples where cal.ef~~lly impected for cracks. In case of the peening pressures with 3 and 4 bar very few cone cracks could be obtained near some single hits. Fig. 6 shows one of the rarely detectable, partially developed conc cracks near a single hit of an alumina sample. 4 Conclusions Shot peening is a common technique to improve the strength of metal components Up to now, ~t has not been successfully apphed to ceramics, as the5e brittle matelrals have been assumed to show no s~gnlficant plast~ defoiinat~on due to mechanical loadmg dnd hence would not de-

6 Figure 6 Paillally developed cone clack in alumma due to a v~iigle hit velop any residual stresses improving the strength. The presented results show however that under specific shot peening conditions also in brittle materials like silicon nitride and alumina high compressive stresses up to more than I GPa can be introduced near the surface. No significant damage is obtained. Exposing these strengthened surfaces to loading situations, which are characterized by a steep near-surface stress gradient, a boost of load capacity and strength can be evaluated. Such loading situation exists in, e.g., roller and sliding bearings or cutting tools. Further investigations will concentrate on increasing the depth of the compressive residual stress field, on the integrity of edges and on the possibility of restoring the strength of >>roughly(( machined ceramics. The patented shot peening proceclure is now going into the first application which is the increase of the load capacity of full ceramic roller bearings. 5 Acknowledgement Part of the investigation was sponsored by the Deutsche Forschungsgeineinschaft (DFG) and by the Bundesministerium fiir Bildung und Forschung (BMRF). 6 References (11 Rombach, M.: Experimentelle Untersuchungen und bruchmechanische Modellierung zum Versagensverhalten einer Siliciumnitridkeran~ik unter Kontaktbeanspruchungen, Dissertation Universilat Karlsruhe, [2] Op de Hipt, M.: Ka~~dzonencliarakterisierung hartbearbeiteter und tribologisch bclastcter A1203 und Sic-Kerarniken, Fortschritt-Berichte VDI Reihe 5 Nr. 521, VDI-Verlag, [3] Miiller, P., Macherauch, E.: Das ~in~~-verfahren der rontgenographischen Spannungsmessung, Z. ang. Phys. 13, 1961,